Herbicide resistant weeds and stink bugs get a lot of ink, but nematodes remain one of the biggest threats to Southeast crop production.
Getting the latest high tech equipment into the hands of farmers at a cost they can afford is an ongoing challenge, says Clemson University Plant Pathologist John Mueller. Like most technology, the cost has come down and the efficiency has gone up, he says.
Managing nematodes is a tough enough challenge when you know what species you have in a field. Trying to figure out which part of a field has high nematode populations requiring nematicide treatment, and which part of the field doesn’t, at one time was virtually impossible.
Mueller has worked for over a decade with fellow Clemson Researchers Ahmad Khalilian and Will Henderson to find ways to adapt data generated by the Veris Soil Electrical Conductivity meter and other technologies for better recognition and management of nematodes in cotton.
The Veris system uses GPS and the technology of soil electrical conductivity (EC) to identify areas of varying soil textures. When the Veris Sensor Cart is pulled through the field, it acquires EC measurements, geo-referencing them using GPS. The data collected by the Sensor Cart is then displayed on an instrument panel, along with its coordinates. The result is a map that identifies the contrasting soil conductivity zones and therefore areas of varying soil texture.
“The bottom line challenge of farmers across the board is to use less pesticide and get better results. Technology allows us to do that, but it’s only good if a farmer can afford to buy it and use it,” Mueller stresses.
With the exception of some of the herbicide resistance, a grower has a chance to impact his yield more with good nematode management than with any other pest. Day in and day out, these tiny little creatures cause as much damage to crops in the Southeast as any other pest, Mueller notes.
In an extensive sampling project, Clemson researchers sampled every field on the farms of four South Carolina growers with 1,500+ acres. In 32 percent of the acreage, we determined there was a minimum 10 percent yield loss due to nematodes, Mueller says.
Visually finding a 10 percent yield loss in cotton is difficult. At 10 percent yield loss levels, you don’t often see stunting or other visual signs of stress to the plant.
Of the fields sampled, 37 percent had 25 percent or higher yield loss. At this level you will see areas of stunted, yellow plants and where bolls don’t open at the end of the year.
Though these samples were taken a few years back, Mueller says he is convinced many cotton fields in South Carolina lose more cotton to nematodes than growers think.
In the past few years soybean acreage has increased across the Southeast. Mueller contends beans may be at even more risk to nematode damage than cotton. Cotton in South Carolina has benefited from a dramatic increase in peanut acreage over the past five years. Nematodes that go to cotton don’t go to peanuts and vice-versa.
It is a myth that corn isn’t affected by nematodes. “We will get yield losses on corn sometimes, but a much bigger problem is that nematodes build up on corn. You might not see much damage to corn, but if you follow corn with a susceptible soybean or cotton variety you may see major problems from nematode damage,” the South Carolina researcher says.
“In South Carolina all our soil types contain the word sandy. Typical South Carolina soils have 80 percent or more sand. Nematodes thrive in sand. If you have sand in the title of your soil, you are going to have moisture stress,” Mueller stresses. “The more stress you have, the more nematode damage you will have,” he adds.
In the past, treating these microscopic pests involved applying a single rate of Temik 15G or Telone II across a field. In reality, some areas of the field have damaging levels of nematodes and some don’t, but all areas of the field get the same nematicide rate.
One alternative is grid sampling. Dividing the field into one-half to one acre sections gives you an average nematode density, and you can apply more or less nematicide based on the average within a grid. “To do a grid sample right takes a lot of time and costs a lot of money, and at the end of the day you still have an average,” according to Mueller.
“The advent of GPS allows us to map a field to less than a foot. We can make a continuous flow map and every few feet you have a new record, and you can know what goes on in every square foot of soil in a field.
“Using a Veris rig, we can tell a grower what percent sand and clay he has at each point in a field. Combining the Veris reading with GPS allows us to map a field for soil texture and know with some certainty where we have high levels of electrical conductivity, i.e. low levels of sand, and a more productive soil type. Low electrical conductivity levels indicate sandy soils which are generally less productive.
“We can take these soil maps and using what we know about nematodes make an accurate prediction of both how many and what kind of nematodes we have in a very small area of a field.
“For example, we know that certain nematodes, like Columbia lance nematodes, because of their large body size, are more common in sandy soils. Other species, like reniform nematodes, prefer the heavier soils that contain a higher percentage of clay.
“You get a double effect with Columbia lance, probably our most common nematode in South Carolina, because these sandier soils are more prone to moisture stress. The combination of water stress and nematode damage is a tough one for crops to survive without yield loss,” Mueller says.
“Most of our work was done on cotton, but knowing what species of nematode is important in developing cost efficient management strategies, regardless of the crop. Ring nematode, which is a common pest in peach orchards, physically can only live in sandy soils. So, if you are planting peach trees, you would only treat the sandy areas of the orchard,” Mueller explains.
In addition to projecting which nematode species is present in what area of a field the Veris-generated soil texture maps also are useful in determining what levels of stress may occur. Each nematode species has different damage thresholds by soil types. Ten sting nematodes, for example, which would only occur in very sandy soils, can cause as much damage as 75 Columbia lance nematodes. By comparison, 75 Columbia lance nematodes can cause as much damage as 250 reniform nematodes.
Where nematode populations and stress are projected to be low a grower can treat with 5-7 pounds of Temik per acre. Where nematode populations are low, as in cotton the year after peanuts, Avicta seed treatment may be sufficient to manage lower levels of nematodes.
If a grower has high levels of sting or Columbia lance nematodes, he may need to go from six pounds of Temik at roughly $18 per acre to three gallons of Telone for $35-$40 per acre. The best option in some cases with heavy nematode pressure comes from rotating to a non-susceptible crop.
“In our most recent tests, using variable rate application, we varied from zero Telone II to four gallons per acre in the same field. Without variable rate technology, the grower would typically treat for the highest nematode populations using the higher rate of nematicide across the entire field. As the nematode pressure goes up in a cotton field and the cost of pesticides goes up, the need for more precise application becomes increasingly cost effective,” Mueller stresses.
“In one field with variable soils and subsequently variable nematode pressure, we increased yield by 55 pounds of lint per acre. But, we only averaged one half gallon of Telone II per acre, versus the standard uniform treatment of three gallons per acre. So, we saved $25 per acre on Telone II costs plus another $25 or so on increased lint yield.
“The cost differential is less dramatic in a Temik 15G-treated field, but still we saw a five percent yield increase and 34 percent lower nematicide usage,” Mueller explains.
“Variable rate application of nematicides is a win/win situation for growers. It is affordable technology that can put money in a grower’s pocket,” Mueller says.
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EDITOR’S NOTE — John Mueller has been a plant pathologist at Clemson University since 1983. He currently heads the South Carolina sentinel system for monitoring the progress of Asian soybean rust. Currently, he is also the Director of Clemson University’s Edisto Agricultural Research and Extension Center near Blackville, S.C.